Laser Chamber, Gas Laser Apparatus, and Electronic Device Manufacturing Method

The present application claims the benefit of Japanese Patent Application No. 2024-207718, filed on Nov. 28, 2024, the entire contents of which are hereby incorporated by reference.

The present disclosure relates to a laser chamber, a gas laser apparatus, and an electronic device manufacturing method.

In recent years, a semiconductor exposure apparatus is required to improve the resolution thereof as semiconductor integrated circuits are increasingly miniaturized and highly integrated. To this end, reduction in the wavelength of light emitted from a light source for exposure is underway. For example, a KrF excimer laser apparatus, which outputs laser light having a wavelength of about 248 nm, and an ArF excimer laser apparatus, which outputs laser light having a wavelength of about 193 nm, are used as a gas laser apparatus for exposure.

The light from KrF and ArF excimer laser apparatuses performing spontaneous laser oscillation has a wide spectral linewidth ranging from 350 to 400 pm. A projection lens made of a material that transmits ultraviolet light, such as the KrF and ArF laser light, therefore produces chromatic aberrations in some cases. As a result, the resolution of the projection lens may decrease. To avoid the decrease in the resolution, the spectral linewidth of the laser light output from the gas laser apparatus needs to be narrow enough to make the chromatic aberrations negligible. To this end, a line narrowing module (LNM) including a line narrowing element (such as etalon or grating) is provided in some cases in a laser resonator of the gas laser apparatus to narrow the spectral linewidth. A gas laser apparatus providing a narrowed spectral linewidth is hereinafter referred to as a narrowed-line gas laser apparatus.

[PTL 1]JP-A-5-333865 [PTL 2]WO 2023/006931

A laser chamber according to an aspect of the present disclosure includes a container, a first electrode, a second electrode, and a sound absorber. The container is filled with a laser gas. The first electrode is disposed in the container. The second electrode is disposed in the container and faces the first electrode. The sound absorber is disposed on at least one of an upstream side and a downstream side of the laser gas from the first electrode. The sound absorber is configured to cause a propagation speed of an acoustic wave generated in a discharge space between the first electrode and the second electrode to decrease as a distance from the discharge space increases.

A gas laser apparatus according to another aspect of the present disclosure includes an optical resonator and a laser chamber. The laser chamber is so disposed that an optical path of the optical resonator passes through the laser chamber. The laser chamber includes a container, a first electrode, a second electrode, and a sound absorber. The container is filled with a laser gas. The first electrode is disposed in the container. The second electrode is disposed in the container and faces the first electrode. The sound absorber is disposed on at least one of an upstream side and a downstream side of the laser gas from the first electrode. The sound absorber is configured to cause a propagation speed of an acoustic wave generated in a discharge space between the first electrode and the second electrode to decrease as a distance from the discharge space increases.

An electronic device manufacturing method according to another aspect of the present disclosure includes: generating laser light by using a gas laser apparatus; outputting the laser light to an exposure apparatus; and exposing a photosensitive substrate to the laser light in the exposure apparatus to manufacture electronic devices. The gas laser apparatus includes an optical resonator and a laser chamber. The laser chamber is so disposed that an optical path of the optical resonator passes through the laser chamber. The laser chamber includes a container, a first electrode, a second electrode, and a sound absorber. The container is filled with a laser gas. The first electrode is disposed in the container. The second electrode is disposed in the container and faces the first electrode. The sound absorber is disposed on at least one of an upstream side and a downstream side of the laser gas from the first electrode. The sound absorber is configured to cause a propagation speed of an acoustic wave generated in a discharge space between the first electrode and the second electrode to decrease as a distance from the discharge space increases.

1. Comparative Example 1.1 Configuration 1.2 Operation 1.3 Problems 2. Embodiment 2.1 Configuration 2.2 Operation 2.3 Advantages 2.4 Output angle of acoustic wave 2.5 Relationship between propagation speed and compressibility 2.6 Relationship between propagation speed and porosity 2.7 Variations 3. Electronic device manufacturing method

An embodiment of the present disclosure will be described below in detail with reference to the drawings. The embodiment described below shows some examples of the present disclosure and is not intended to limit the contents of the present disclosure. Furthermore, all configurations and operations described in the embodiment are not necessarily essential as configurations and operations in the present disclosure. Note that the same element has the same reference character, and no duplicate description of the same element will be made.

Comparative Example of the present disclosure will first be described. Comparative Example of the present disclosure is an aspect that the applicant is aware of as known only by the applicant, and is not a publicly known example that the applicant is self-aware of.

2 2 2 2 1 2 FIGS.and 1 FIG. 2 FIG. 1 FIG. The configuration of a gas laser apparatusaccording to Comparative Example will be described with reference to.schematically shows the configuration of the gas laser apparatus.is a cross-sectional view of the gas laser apparatusshown inand viewed in a Z direction. The gas laser apparatusis a discharge-excitation gas laser apparatus that excites a laser gas through discharge, and is, for example, an excimer laser apparatus.

1 FIG. 2 It is assumed inthat a traveling direction of pulse laser light PL output from the gas laser apparatusis the Z direction. It is further assumed that a discharge direction that will be described later is a Y direction. It is further assumed that the direction orthogonal to the Z and Y directions is an X direction. Note that the pulse laser light PL is an example of “laser light” according to the technology described in the present disclosure.

2 10 11 12 13 14 17 15 16 The gas laser apparatusis a narrowed-line gas laser apparatus including a laser chamber, a charger, a pulse power module (PPM), a pulse energy measuring unit, a processor, a pressure sensor, and a laser resonator. A line narrowing moduleand an output coupling mirrorconstitute the laser resonator.

10 10 20 21 22 23 24 28 28 29 29 30 10 30 31 32 33 a a b a b a 1 2 FIGS.and The laser chamberincludes, for example, a containermade of aluminum metal and having a surface plated with nickel. A primary electrode, a ground plate, a return plate, a fan, a heat exchanger, insulating guidesand, sound absorbersand, and a preliminary ionization electrodeare provided inside the container, as shown in. The preliminary ionization electrodeincludes a preliminary ionization outer electrode, a dielectric pipe, and a preliminary ionization inner electrode.

10 a A laser gas containing fluorine is encapsulated as a laser medium in the container. The laser gas includes, for example, argon, krypton, xenon, or any other element as a rare gas, neon, helium, or any other element as a buffer gas, and fluorine, chlorine, or any other element as a halogen gas.

10 26 25 10 12 26 10 a a a The containerfurther has an opening. An electrically insulating plate, in which feedthroughsare embedded, is attached to the containervia an O-ring that is not shown so as to close the opening. The PPMis disposed on the electrically insulating plate. The containeris grounded.

12 20 25 12 20 11 12 20 The PPMincludes a charging capacitor that is not shown and is connected to the primary electrodevia the feedthroughs. The PPMincludes a switch SW, which causes the primary electrodeto perform discharge. The chargeris connected to the charging capacitor of the PPM. The discharge that occurs at the primary electrodeis hereinafter referred to as primary discharge.

20 20 20 20 20 10 20 20 27 20 20 a b a b a a b a b The primary electrodeincludes a cathode electrodeand an anode electrode. The cathode electrodeand the anode electrodeare so disposed that the discharge surfaces thereof face each other in the container. The space between the discharge surface of the cathode electrodeand the discharge surface of the anode electrodeis called a discharge space. The cathode electrodeand the anode electrodeeach extend in the Z direction.

20 26 25 20 10 20 20 20 20 21 20 20 a a a b a b b a b The surface of the cathode electrodethat is opposite to the discharge surface thereof is supported by the electrically insulating plate, and is connected to the feedthroughs. That is, the cathode electrodeis disposed at a position closer to the inner wall of the containerthan the anode electrodewith the cathode electrodefacing the anode electrode. The surface of the anode electrodethat is opposite to the discharge surface thereof is supported by the ground plate. The cathode electrodeand the anode electrodeare examples of “first electrode and second electrode” according to the technology described in the present disclosure.

21 10 22 10 21 22 21 10 a a a. The ground plateis connected to the containervia the return plate. The containeris grounded. The ground plateis therefore grounded via the return plate. Ends of the ground platein the Z direction are fixed to the container

23 10 21 27 23 23 10 a a a. The fanis a crossflow fan used to circulate the laser gas in the container, and is disposed on the side of the ground platethat is opposite to the discharge space. A motor, which rotationally drives the fan, is connected to the container

23 27 27 27 23 24 24 24 The laser gas blown out from the fanflows into the discharge space. A flowing direction of the laser gas flowing into the discharge spaceis substantially parallel to the X direction. The laser gas flowing out of the discharge spaceis suctioned into the fanvia the heat exchanger. The heat exchangerchanges the temperature of the laser gas by performing heat exchange between a refrigerant supplied into the heat exchangerand the laser gas.

28 28 26 27 20 28 20 28 20 a b a a a b a. The insulating guidesandare disposed at a surface of the electrically insulating platethat is closer to the discharge spaceso as to sandwich the cathode electrode. The insulating guideis disposed on the upstream side of the laser gas from the cathode electrode. The insulating guideis disposed on the downstream side of the laser gas from the cathode electrode

28 28 23 20 20 28 28 26 a b a b a b 2 3 The insulating guidesandare shaped so as to guide the flow of the laser gas so that the laser gas from the fanefficiently flows between the cathode electrodeand the anode electrode. The insulating guidesandand the electrically insulating plateare made, for example, of a ceramic material such as alumina (AlO), which has low reactivity with fluorine gas.

29 29 21 27 20 29 20 29 20 a b b a b b b. The sound absorbersandare disposed at a surface of the ground platethat is closer to the discharge spaceso as to sandwich the anode electrode. The sound absorberis disposed on the upstream side of the laser gas from the anode electrode. The sound absorberis disposed on the downstream side of the laser gas from the anode electrode

29 29 29 29 29 29 23 20 20 a b a b a b a b The sound absorbersandare made, for example, of a porous material. The porous material of which the sound absorbersandare made is at least one of a porous metal, a foamed metal, and a mesh metal. The materials described above are metallic materials having low reactivity with the laser gas, for example, at least one of nickel, aluminum, and copper. The sound absorbersandguide the laser gas from the fanto flow efficiently between the cathode electrodeand the anode electrode, and absorb an acoustic wave generated by the primary discharge.

18 18 10 18 18 a b a b A laser gas supplierand a laser gas dischargerare connected to the laser chamber. The laser gas supplierincludes a valve and a flow rate control valve, and is connected to a gas cylinder containing the laser gas. The laser gas dischargerincludes a valve and a discharge pump.

19 19 10 10 10 27 19 19 a b a a a b. Windowsandare provided at ends of the containerto cause light generated in the containerto exit out thereof. The laser chamberis so disposed that the optical path of the optical resonator passes through the discharge spaceand the windowsand

15 15 15 15 10 19 15 a b a a b. The line narrowing moduleincludes a prismand a grating. The prismincreases the beam width of the light output from the laser chambervia the window, and transmits the light toward the grating

15 15 15 15 15 10 15 b b b b b a The gratingis disposed in the Littrow arrangement, which causes the angle of incidence of the light incident on the gratingto be equal to the angle of diffraction of the light diffracted by the grating. The gratingis a wavelength selection element that selectively extracts light having a specific wavelength and wavelengths in the vicinity thereof in accordance with the angle of diffraction. The light that returns from the gratingto the laser chambervia the prismhas a narrowed spectral width.

16 10 19 10 16 b The output coupling mirrortransmits part of the light output from the laser chambervia the windowand reflects the other part of the light back into the laser chamber. The surfaces of the output coupling mirrorare each coated with a partially reflective film.

10 15 16 27 16 The light output from the laser chambertravels back and forth between the line narrowing moduleand the output coupling mirrorand is amplified whenever passing through the discharge space. Part of the amplified light is output as the pulse laser light PL via the output coupling mirror. The wavelength of the pulse laser light PL falls within an ultraviolet range from 150 nm to 380 nm, and is, for example, the wavelength of light from an excimer laser apparatus that performs laser oscillation.

13 16 13 13 13 13 a b c. The pulse energy measuring unitis disposed in the optical path of the pulse laser light PL output via the output coupling mirror. The pulse energy measuring unitincludes a beam splitter, a light collection optical system, and a photosensor

13 13 13 13 13 13 14 a b b a c c The beam splittertransmits the pulse laser light PL at high transmittance and reflects part of the pulse laser light PL toward the light collection optical system. The light collection optical systemcollects the light reflected off the beam splitterat the light receiving surface of the photosensor. The photosensormeasures the pulse energy of the light collected at the light receiving surface, and outputs the measured value to the processor.

17 10 14 14 10 11 a a The pressure sensordetects the gas pressure in the containerand outputs the detected value to the processor. The processordetermines the gas pressure of the laser gas in the containerbased on the detected value of the gas pressure and the charging voltage applied by the charger.

11 12 12 14 12 20 The chargeris a high voltage power supply that supplies the charging voltage to the charging capacitor incorporated in the PPM. The switch SW in the PPMis controlled by the processor. When the switch SW is turned on from the state in which the switch SW is off, the PPMgenerates high voltage pulses from the electrical energy stored in the charging capacitor and applies the pulses to the primary electrode.

14 110 100 100 110 14 The processoris a CPU (central processing unit) or any other processing device that transmits and receives various signals to and from an exposure apparatus controllerprovided in an exposure apparatus. For example, target pulse energy, an oscillation trigger signal, and other factors of the pulse laser light PL output to the exposure apparatusare transmitted from the exposure apparatus controllerto the processor.

14 2 110 The processorharmoniously controls the operation of each element of the gas laser apparatusbased on the various signals transmitted from the exposure apparatus controller, the measured value of the pulse energy, the detected value of the gas pressure, and other pieces of information.

2 15 Note that the gas laser apparatusis not necessarily limited to a narrowed-line laser apparatus, and may instead be a laser apparatus that outputs spontaneously oscillating light. For example, the line narrowing modulemay be replaced with a highly reflective mirror.

2 14 18 10 10 23 23 10 a a a a 2 FIG. The operation of the gas laser apparatusaccording to Comparative Example will next be described. The processorfirst controls the laser gas supplierto cause it to supply the laser gas into the containerof the laser chamber, and drives the motorto rotate the fan. The laser gas filled in the containerthus circulates, as indicated by the arrows in.

14 110 2 The processorreceives the target pulse energy and the oscillation trigger signal transmitted from the exposure apparatus controller. Note that the oscillation trigger signal is a signal that instructs the gas laser apparatusto output the pulse laser light PL corresponding to one pulse.

14 11 14 12 The processorsets a charging voltage corresponding to the target pulse energy in the charger. The processoroperates the switch SW in the PPMin synchronization with the oscillation trigger signal.

12 33 31 30 20 20 30 27 a b When the switch SW in the PPMis turned on from the state in which the switch SW is off, the voltage is applied to the space between the preliminary ionization inner electrodeand the preliminary ionization outer electrodeof the preliminary ionization electrode, and to the space between the cathode electrodeand the anode electrode. Corona discharge thus occurs at the preliminary ionization electrode, and UV (ultraviolet) light is generated. Irradiating the laser gas in the discharge spacewith the UV light preliminarily ionizes the laser gas.

20 20 27 20 20 27 a b a b Thereafter, when the voltage between the cathode electrodeand the anode electrodereaches the dielectric breakdown voltage, the primary discharge occurs in the discharge space. Assuming that the discharge direction of the primary discharge is the direction in which the electrons flow, the discharge direction is the direction from the cathode electrodetoward the anode electrode. When the primary discharge occurs, the laser gas in the discharge spaceis excited and emits light.

15 16 15 16 The light emitted from the laser gas is reflected off the line narrowing moduleand the output coupling mirrorand travels back and forth in the laser resonator, so that laser oscillation occurs. The light having a bandwidth narrowed by the line narrowing moduleis output as the pulse laser light PL via the output coupling mirror.

16 13 13 14 Part of the pulse laser light PL output via the output coupling mirrorenters the pulse energy measuring unit. The pulse energy measuring unitmeasures the pulse energy of the incident pulse laser light PL, and outputs the measured value to the processor.

14 14 The processorcalculates a difference ΔE between the measured pulse energy and the target pulse energy. The processorperforms feedback control on the charging voltage in such a way that the difference ΔE approaches zero.

14 18 10 14 18 10 a a b a The processorcontrols the laser gas supplierto cause it to supply the laser gas into the containeruntil a predetermined pressure is reached when the charging voltage becomes higher than the maximum value of an allowable range. The processorcontrols the laser gas dischargerto cause it to discharge the laser gas from the containeruntil the predetermined pressure is reached when the charging voltage becomes lower than the minimum value of the allowable range.

13 100 The pulse laser light PL having passed through the pulse energy measuring unitenters the exposure apparatus.

10 27 27 23 24 24 a Note that discharge products are generated in the containerby the primary discharge in the discharge space. The generated discharge products are moved away from the discharge spaceby the gas flow generated by the fan. The discharge is thus stabilized. Furthermore, the temperature of the laser gas increases due to the primary discharge. The laser gas having the increased temperature is cooled by cooling water flowing in the heat exchangerin the course of passing through the heat exchanger.

20 20 29 29 a b a b. Furthermore, the primary discharge at the cathode electrodeand the anode electrodegenerates the acoustic wave. The generated acoustic wave is attenuated when absorbed by the sound absorbersand

3 FIG. 3 FIG. 3 FIG. 20 10 27 27 10 27 27 a schematically shows a portion including the primary electrodein the laser chamberaccording to Comparative Example. The broken lines shown indiagrammatically show the acoustic wave generated by the primary discharge. As shown in, the acoustic wave generated by the primary discharge is a compressional wave of the laser gas and propagates while spreading from the discharge space. The acoustic wave may, at the timing when the primary discharge occurs next, remain in the discharge spaceor be reflected off a structure in the containerand may return to the discharge space. Primary discharge that occurs when the acoustic wave in the discharge spaceis not attenuated as described above may adversely affect the laser performance such as the spectral linewidth of the pulse laser light PL.

2 29 29 27 29 29 27 27 a b a b In the gas laser apparatusaccording to Comparative Example, the sound absorbersandare provided to attenuate the acoustic wave in the discharge space. However, part of the acoustic wave may return from the sound absorbersandinto the discharge space, and hinder the attenuation of the acoustic wave in the discharge space.

4 FIG. 4 FIG. 29 1 29 29 29 29 29 21 b b b b b b diagrammatically shows the state in which the acoustic wave enters the sound absorber. A reference character Arepresents a propagation direction of the acoustic wave that enters the sound absorber. A reference character WF represents the wave front of the acoustic wave. For example, the acoustic wave is incident as a plane wave on the surface of the sound absorberat right angles, as shown in. The acoustic wave propagates in the sound absorberat a uniform speed, so that the acoustic wave having entered the sound absorberpropagates in the sound absorberwith no phase difference produced and is reflected off the ground plate.

5 FIG. 29 2 29 21 29 29 29 27 b b b b b diagrammatically shows the state in which the acoustic wave exits out of the sound absorber. A reference character Arepresents the propagation direction of the acoustic wave that exits out of the sound absorber. The acoustic wave reflected off the ground platepropagates in the sound absorberwith no phase difference produced, and exits in the direction perpendicular to the surface of the sound absorber. As described above, since the acoustic wave exits in the direction perpendicular to the surface of the sound absorber, part of the acoustic wave returns into the discharge spaceto hinder the attenuation of the acoustic wave.

4 5 FIGS.and 29 29 b a. show the state in which the acoustic wave enters and exits out of the sound absorber, and the same holds true for the sound absorber

27 In particular, the higher the repetition frequency of the pulse laser light PL, the more insufficient the attenuation of the acoustic wave in the discharge spaceuntil the timing of the subsequent primary discharge, so that the laser performance deteriorates.

27 To solve the problem described above, an object of the present disclosure is to enhance the laser performance by attenuating the acoustic wave in the discharge spaceearly.

2 2 10 A gas laser apparatusaccording to an embodiment of the present disclosure is configured in the same manner as the gas laser apparatusaccording to Comparative Example except a different configuration in the laser chamber.

6 FIG. 6 FIG. 20 10 40 40 29 29 40 40 21 27 20 a b a b a b b schematically shows a portion including the primary electrodein the laser chamberaccording to the embodiment. The broken lines shown indiagrammatically show the acoustic wave generated by the primary discharge. In the present embodiment, sound absorbersandare provided in place of the sound absorbersand. The sound absorbersandare disposed at a surface of the ground platethat is closer to the discharge spaceso as to sandwich the anode electrode, as in Comparative Example.

40 40 40 40 a b a b The sound absorbersandare made, for example, of a porous material, as in Comparative Example. The material of which the sound absorbersandare made is at least one of a porous metal, a foamed metal, and a mesh metal. The materials described above are metallic materials having low reactivity with the laser gas, for example, at least one of nickel, aluminum, and copper.

40 40 40 40 27 40 40 a b a b a b In the present embodiment, the sound absorbersandare so configured that the propagation speed of the acoustic wave continuously changes in the X direction, which is a direction perpendicular to the discharge direction. Specifically, in the sound absorbersand, the propagation speed of the acoustic wave decreases as the distance from the discharge spaceincreases. In the sound absorber, the sound wave propagates at a lower speed on the upstream side of the laser gas than on the downstream side of the laser gas. In the sound absorber, the sound wave propagates at a lower speed on the downstream side of the laser gas than on the upstream side of the laser gas.

40 40 40 40 40 40 a b a b a b The propagation speed of the acoustic wave in the sound absorbersandcan be changed by changing the compressibility of the sound absorbersand. This is because the porosity in the sound absorbersandchanges in accordance with the compressibility. The greater the compressibility, the smaller the porosity and the lower the propagation speed of the acoustic wave.

40 40 40 40 a b a b The propagation speed of the acoustic wave in the sound absorbersandcan also be changed by changing the porosity of multiple cell structures contained in the sound absorbersand. The smaller the porosity, the lower the propagation speed of the acoustic wave. Changing the porosity corresponds to changing the density.

40 40 a b Note that the propagation speed of the acoustic wave can also be changed by changing the elastic modulus in the sound absorbersand. The smaller the elastic modulus, the lower the propagation speed of the acoustic wave.

2 40 40 a b The operation of the gas laser apparatusaccording to the present embodiment is the same as that in Comparative Example except that the sound absorbersandattenuate the acoustic wave differently.

7 FIG. 7 FIG. 40 1 40 40 40 27 40 40 21 b b b b b b diagrammatically shows the state in which the acoustic wave enters the sound absorber. The reference character Arepresents the propagation direction of the acoustic wave that enters the sound absorber. For example, the acoustic wave is incident as a plane wave on the surface of the sound absorberat right angles, as shown in. In the present embodiment, since the propagation speed of the acoustic wave in the sound absorberdecreases as the distance from the discharge spaceincreases, the acoustic wave entering the sound absorberpropagates with a phase difference produced in the sound absorberwhile the wave front WF inclines, and is reflected off the ground plate.

8 FIG. 40 2 40 21 40 40 40 27 b b b b b diagrammatically shows the state in which the acoustic wave exits out of the sound absorber. The reference character Arepresents the propagation direction of the acoustic wave that exits out of the sound absorber. The acoustic wave reflected off the ground platepropagates in the sound absorberwith a phase difference produced while the wave front WF inclines, and exits via the surface of the sound absorber. As described above, in the present embodiment, the acoustic wave exiting out of the sound absorberobliquely propagates in a direction away from the discharge space.

7 8 FIGS.and 40 40 40 27 b a a and subsequent figures will be described with reference to the sound absorber, and the same holds true for the sound absorber. That is, the acoustic wave exiting out of the sound absorberobliquely propagates in a direction away from the discharge space.

40 40 27 27 27 a b In the present embodiment, the acoustic wave that enters and exits out of the sound absorbersandobliquely propagates in a direction away from the discharge space, preventing the acoustic wave from returning to the discharge space. The acoustic wave in the discharge spacecan therefore be attenuated early, so that the laser performance can be improved.

27 As described above, according to the present embodiment, since the acoustic wave in the discharge spacecan be attenuated early, the laser performance can be enhanced even at a high repetition frequency, for example, higher than or equal to 6 kHz.

40 40 2 40 1 40 40 40 a b b b b b. 9 FIG. The output angle of the acoustic wave output from the sound absorbersandwill next be described.shows an angle θ of the propagation direction Aof the acoustic wave output from the sound absorberwith respect to the propagation direction Aof the acoustic wave entering the sound absorber. The angle θ represents the angle at which the acoustic wave having entered the sound absorberexits out of the sound absorber

40 40 40 40 40 b b b b b 10 FIG. Let T be the length of the sound absorberin the Y direction, L be the length of the sound absorberin the X direction, Cmax be the maximum propagation speed of the acoustic wave in the sound absorber, and Cmin be the minimum propagation speed of the acoustic wave in the sound absorberas shown in, and the angle θ is calculated by Expression (1) below. Note that the acoustic wave has a component that propagates in the X direction as well as in the Y direction, but the propagation speed of the acoustic wave in the sound absorberis defined by the propagation speed in the Y direction.

40 b The angle θ calculated by Expression (1) described above corresponds to the inclination angle of the wave front WF caused by the propagation of the sound wave in the sound absorber. The length T in the Y direction is hereinafter referred to as a thickness T.

27 40 40 b a. To attenuate the acoustic wave in the discharge spaceearly, the angle θ is preferably greater than or equal to 20° but smaller than 90°, and more preferably, 45°. The thickness T of the sound absorberis preferably greater than or equal to 3 mm but smaller than or equal to 10 mm. The same holds true for the sound absorber

40 40 a b When the sound absorbersandare so configured that the compressibility changes, the angle θ described above can be determined based on the compressibility.

11 FIG. 11 FIG. 40 40 27 27 a b shows an example of the relationship between the propagation speed and the compressibility. The higher the compressibility, the lower the propagation speed. To set θ=45° when T=5 mm and L=20 mm, Cmax/Cmin in Expression (1) described above may be set at three. In the example shown in, for each of the sound absorbersand, Cmax/Cmin can be set at three by setting the compressibility at an end closer to the discharge spaceat one and the compressibility at an end farther from the discharge spaceat three. The compressibility of one indicates an uncompressed state.

12 FIG. 40 50 50 40 50 b b shows an example of a method for producing the sound absorberso configured that the compressibility changes. A trapezoidal sound absorberhaving a continuously changing thickness is first produced. The sound absorberis then compressed so as to have a uniform thickness. As a result, the sound absorberin which the compressibility continuously changes and the propagation speed of the acoustic wave continuously changes is produced. The difference in thickness of the sound absorberbefore the compression allows the angle θ described above to be set at a desired value.

13 FIG. 40 51 51 40 40 40 b b b b shows another example of the method for producing the sound absorberso configured that the compressibility changes. A rectangular sound absorberhaving a uniform thickness is first produced. The sound absorberis then compressed so as to have a continuously varying thickness. As a result, the sound absorberin which the compressibility continuously changes and the propagation speed of the acoustic wave continuously changes is produced. The difference in thickness of the sound absorberafter the compression allows the angle θ described above to be set at a desired value. In this case, the sound absorberhas a trapezoidal shape.

12 FIG. 13 FIG. 50 50 In the method shown in, since the sound absorberbefore the compression has a trapezoidal shape, it takes a longer period and requires more effort to produce the sound absorberbefore the compression than in the method shown in, but the sound absorber only needs to be so compressed that the thickness becomes uniform, so that the process involving the compression is readily carried out.

14 FIG. 40 61 68 61 68 61 68 40 40 61 68 b b b shows another example of the method for producing the sound absorberso configured that the compressibility changes. Multiple divided sound absorberstoeach having compressibility different from those of the others are first produced. The divided sound absorberhas the lowest compressibility, and the divided sound absorberhas the highest compressibility. The multiple divided sound absorberstoare then so arranged and joined to each other that the compressibility sequentially changes. As a result, the sound absorberin which the compressibility changes is produced. In this case, in the sound absorber, the compressibility changes stepwise, and the propagation speed of the sound wave changes stepwise. The difference in compressibility between the divided sound absorberand the divided sound absorberallows the angle θ described above to be set at a desired value.

14 FIG. In the example shown in, eight divided sound absorbers are used to form one sound absorber, but the number of the divided sound absorbers is not limited to a specific number and only needs to be two or more. The divided sound absorbers mean a small unit of absorbers that form a single sound absorber in which the propagation speed of the acoustic wave changes.

40 40 a b When the sound absorbersandare so configured that the porosity changes, the angle θ described above can be determined based on the porosity.

15 FIG. 15 FIG. 40 40 27 27 a b shows an example of the relationship between the propagation speed and the porosity. The higher the porosity, the higher the propagation speed. To set θ=450 when T=5 mm and L=20 mm, Cmax/Cmin in Expression (1) described above may be set at three. In the example shown in, for each of the sound absorbersand, Cmax/Cmin can be set at three by setting the porosity at an end closer to the discharge spaceat 0.96 and the porosity at an end farther from the discharge spaceat 0.88. A porosity of 1 indicates that the entire sound absorber is configured with a pore.

16 FIG. 16 FIG. 40 40 27 40 b b b shows an example of a method for producing the sound absorberso configured that the porosity changes. The sound absorberis an aggregate of cell structures each having a pore, as shown in. The cell structures are each configured by joining multiple struts to each other. The porosity can be changed by changing the width of each of the struts. The larger the width of each of the struts, the smaller the porosity. Therefore, by gradually increasing the width of each of the struts as the distance from the discharge spaceincreases, the sound absorberin which the porosity continuously changes and the propagation speed of the acoustic wave changes is produced.

40 40 20 40 40 20 a b b a b b In the embodiment described above, the sound absorbersandare provided so as to sandwich the anode electrode, but only one of the sound absorbersandmay instead be provided. That is, the sound absorber according to the embodiment described above only needs to be provided on at least one of the upstream side of the laser gas from the anode electrodeand the downstream side of the laser gas therefrom.

40 40 21 20 26 20 28 28 27 a b b a a b Furthermore, in the embodiment described above, the sound absorbersandare provided at the ground plate, which supports the anode electrode, but similar sound absorbing members may instead be provided at the electrically insulating plate, which supports the cathode electrode. That is, the sound absorbers according to the embodiment described above may be provided in place of the insulating guidesand. Note in this case that it is preferable to provide insulating sound absorbers. The acoustic wave in the discharge spacecan thus be attenuated earlier.

17 FIG. 100 100 104 106 104 104 2 106 schematically shows an example of the configuration of the exposure apparatus. The exposure apparatusincludes an illumination optical systemand a projection optical system. The illumination optical systemilluminates a reticle pattern of a reticle that is not shown but is placed on a reticle stage RT, for example, with the pulse laser light PL having entered the illumination optical systemfrom the gas laser apparatus. The projection optical systemperforms reduction projection on the pulse laser light PL having passed through the reticle to bring the pulse laser light PL into focus at a workpiece that is not shown but is placed on a workpiece table WT. The workpiece is a photosensitive substrate onto which a photoresist has been applied, such as a semiconductor wafer.

100 The exposure apparatussynchronously translates the reticle stage RT and the workpiece table WT to expose the workpiece to the pulse laser light PL having reflected the reticle pattern. Semiconductor devices can be manufactured by transferring the reticle pattern onto the semiconductor wafer in the exposure step described above and then carrying out multiple other steps. The semiconductor devices are an example of the “electronic devices” in the present disclosure.

2 Note that the gas laser apparatusdoes not necessarily manufacture electronic devices, and can also be used to perform laser processing such as drilling.

The description above is intended to be illustrative and the present disclosure is not limited thereto. Therefore, it would be obvious to those skilled in the art that various modifications to the embodiments of the present disclosure would be possible without departing from the spirit and the scope of the appended claims.

The terms used throughout the present specification and the appended claims should be interpreted as non-limiting terms. For example, terms such as “comprise”, “include”, “have”, and “contain” should not be interpreted to be exclusive of other structural elements. Further, indefinite articles “a/an” described in the present specification and the appended claims should be interpreted to mean “at least one” or “one or more”. Further, “at least one of A, B, and C” should be interpreted to mean any of A, B, C, A+B, A+C, B+C, and A+B+C as well as to include combinations of any thereof and any other than A, B, and C.